Page:Proceedings of the Royal Society of London Vol 69.djvu/364

348 Tmesipteris with three synangia or sporangiophores, with the sporo- phyll of Cheirostrobus 1 There is, indeed, considerable difference in details ; thus the division of the sterile segment in Cheirostrobus is palmatifid, whereas the division of the Tmesipteris sporophyll is dichotomous. But this difference is probably of little importance, for in any case we cannot expect to show more than a general corre- spondence. It is much that we can show in each case an elaborate sporophyll, with three ventral sporangiophores.

Cheirostrobus, although a synthetic form, had probably the most complex cone known amongst the Pteridophytes, and this was doubt- less specialised in some particulars. It may be added that the number of sterile segments and sporangiophores in Cheirostrobus might be reduced to two, and we may compare this with those variations of the sporophylls of Tmesipteris in which only two sporangiophores occur. Further, the vascular bundle of each sterile segment bifurcates at the base of the lamina, and the branches run out into two up-turned processes, that is, the last division of the bundles is dichotomous.

If we admit the homologies suggested here, we may draw an in- teresting parallel between Tmesipteris and the Sphenophyllales on the one hand, and such a simple Lycopodium as L. selago and Lepido- dendron on the other hand. Tmesipteris arid L. xelago are both rela- tively small forms, which show an alternation of zones of sterile and fertile leaves, whereas the Paleozoic plants both showed more complex vegetative structure, with secondary increase and specialised cones.

There are certain other points, hoAvever, to be considered before the affinity between Tmesipteris and the extinct Sphenophyllales can be considered as established. As regards the anatomical structure, the difficulty does not appear to be great. I do not propose to enter fully into this question at present ; it will suffice to quote Scott and Seward on this point. The former states that the Psiloteae " are anatomically perhaps the nearest to the Sphenophylleee," whilst Seward considers that " the anatomical characters of the Sphenophyllum shown are such as one finds in some recent genera of the Lycopodinse, espe- cially Psilotum." But Psilotum is unquestionably closely allied to Tmesipteris.

The character in which a greater contrast exists is the arrangement of the leaves. In the Sphenophyllales the arrangement of the leaves, both vegetative and fertile, in whorls, is a striking feature, whereas in the Psilotese the leaves are scattered. This difference, however, can hardly outweigh the evidence afforded by the other characters.

The Spenophyllales have been recognised as intermediate in their characters between the Equisetales and the Lycopodiales ; even on the assumption, therefore, that the Psilotese are nearer the Sphenophyllales, it would not be surprising to find they possess some Lycopodinous characters.